The present invention relates to a method for supplying a vacuum evaporation material to a position to be heated in forming a thin film on a substrate.
In order to form a thin film on a long film and manufacture a functional film serving as the material of a capacitor, a magnetic tape or the like by means of vacuum evaporation, it is necessary to generate a large quantity of vapor over a long period of time. To this end, it is necessary to successively supply a vacuum evaporation material to a position to be heated where a member such as an evaporation crucible is located.
Conventionally, several methods as shown below have been used to successively supply a vacuum evaporation material.
According to the first method, as shown in "Thin Film Handbook" (published by Ohm Publishing Co., Ltd. in 1983, page 105) edited by Japan Science Promotion Association, a vacuum evaporation material in the form of a wire is supplied to a crucible.
This method is applicable to a vacuum evaporation material of a ductile material such as Al, Ni, or Cu which can be easily formed into a wire. But this method has a problem in that it is very difficult to form a brittle material such as Cr into a wire. In addition, a rigid material such as a material of a magnetic film, for example an alloy of Co-Cr or an alloy of Co-Cr-Ni, is not brittle and could be formed into a wire, but cannot be easily processed, so that the manufacturing cost is high and not of practical use.
According to the second method, the brittle and rigid materials as described above are supplied in the form of a bar.
The problem with the supply of a vacuum evaporation material in the form of a bar is described using FIGS. 1 through 7. In FIG. 1, reference numeral 1 denotes a vacuum evaporation material, accommodated in a crucible 2, which is heated by electron beams scanned in the direction, for example, shown by the arrow (E) and fuses. Reference numeral 3 designates a bar-shaped long supply vacuum evaporation material, (hereinafter referred to as bar material) 11, 12, 13 denote rollers for guiding the bar material 3, and 14 denotes a driving roller. A motor 20 is installed either in a vacuum chamber (not shown), thus directly driving the driving roller 14 or on the outside, thus driving the driving roller 14 through a known rotation transmitting unit. The driving roller 14 is driven by the motor 20 in CCW direction, and the driving roller 14 and the rotation roller 11 sandwich the bar material 3 therebetween, the driving roller 14 thus feeding the bar material 3 in the direction shown by the arrow (A) at a constant speed from above the liquid surface 4 toward the liquid surface 4 of the vacuum evaporation material 1, which has fused. Reference numerals 15 and 16 denote covers for preventing the vapor of the vacuum evaporation material 1 from adhering to the rollers 11, 12, 13, and 14.
In order to form a thin film 9 on a substrate 8 positioned above the crucible 2 over a long period of time by the above construction, the bar material 3 is supplied to supplement a vaporization-reduced amount of the vacuum evaporation material 1 accommodated in the crucible 2.
The behavior of the end 5 of the bar material 3 which takes place after the end 5 contacts the liquid surface 4 is described based on FIGS. 2 through 5. FIG. 2 is a view showing the end 5 of the bar material 3, which has just contacted the liquid surface 4, and this condition is supposed to be condition (a). In this condition (a), the end 5 starts melting as a result of heat absorption from the liquid surface 4, thus fusing into a solution 6 of the vacuum evaporation material 1 as shown by the arrow (R) of condition (b) of FIG. 3. At this time, heat is transmitted to the end 5 in the direction opposite to the direction shown by the arrow (R), and consequently the fusion of the end 5 progresses. With the progress of the fusion, the distance (g) between the liquid surface 4 and the end 5 of the bar material 3 becomes long, so that it becomes difficult for heat to be transmitted from the liquid surface 4 to the end 5. As a result, the fusion amount of the end 5 is reduced as shown by condition (c) of FIG. 4, and the amount of the solution 6 becomes small. Consequently, it becomes more difficult for the end 5 to melt, with the result that the fusion stops as shown by condition (d) of FIG. 14. The bulge of the end 5 of FIG. 5 is a droplet 6A of the solution 6.
Meanwhile, the bar material 3 is continuously fed in the direction shown by the arrow (A), but the fusion speed of the end 5 from the condition (a) until the condition (d) is faster than the bar material feeding speed. Therefore, a gap (D) is formed as shown by the condition (d). Since the bar material 3 is continuously fed, the gap (D) becomes small with the elapse of time, thus resulting in the condition (a). Thus, this cycle is repeated. That is, although the bar material 3 is continuously fed, the bar material 3 is intermittently fed to the liquid surface 4.
While the end 5 is fusing with the liquid surface, the end 5 absorbs heat from the liquid surface 4, so that the temperature of the liquid surface 4 lowers. As a result, the evaporation speed lowers and the evaporation speed cyclically fluctuates with the elapse of time, as shown in FIG. 6. The distance (D) of the condition (d) becomes great as the diameter of the bar material 3 becomes large. The reason is that, supposing that the configuration of the solution 6 is substantially a cylinder, as the diameter of the cylinder becomes large, the ratio of surface area of cylinder to volume of cylinder becomes small. Therefore, the ratio of heat quantity which escapes from the surface of the cylinder by radiation to the heat quantity which is transmitted from the liquid surface 4 to the top end 5 through the solution 6 becomes small, so that it takes long for the end 5 to fuse. Accordingly, when the diameter of the bar material becomes large, both the cycle T.sub.L and fluctuation width H.sub.L of the evaporation speed shown in FIG. 6 become large.
This method has a problem in that due to the fluctuation of the evaporation speed, the film thickness is not uniformly formed in the direction in which vacuum evaporation is carried out while a substrate 8, such as a film, is moving above the crucible 2, forming a film while the substrate 8 is moving.
In order to overcome the above-described problem, according to the third method shown in FIG. 7, the bar material is supplied to the crucible after it is fused. This method is described using the reference numerals of FIG. 1 concerning the same parts of FIG. 7. In FIG. 7, electron beams 7B for fusing the bar material are irradiated onto the end 5 of the bar material 3 to fuse, and unlike the second method, the temperature of the liquid surface 4 is not lowered by the supply of the solution 6 to the liquid surface 4, so that the evaporation speed does not fluctuate. Accordingly, if a film is formed by traveling the substrate 8 above the crucible 2 in a direction perpendicular to the sheet surface of FIG. 7, the film thickness of a thin film 9 does not become nonuniform.
Next, a description is made with regard to a condition for constantly maintaining the composition ratio of an alloyed thin film formed on a substrate when the vacuum evaporation material 1 consists of an alloy of Co-Cr, which is a material, such as a magnetic material, composed of components different in evaporation speeds. The evaporation speed of Cr is faster than that of Co by three to four times, so that supposing that the content of Cr of a thin film to be formed is (M) and the content of Cr of a thin film material 1 is (Y), it is required that M is 3 to 4 times more than Y, that is, M=(3.about.4)Y. The content of Cr of vapor generated from the liquid surface 4 is equal to (M). Accordingly, if a material with Cr content of (M) and equal to an evaporated amount can be supplied, a thin film with Cr content of (M) can be continuously formed with the amount of the thin film material 1 in the crucible 2 kept to be constant.
However, in the construction of FIG. 7, the solution 6 formed by the irradiation of the electron beams 7B onto the end 5 of the bar material 3, with a Cr content of (M), stays at the end 5 of the bar material 3 by its surface tension, thus becoming a large droplet 6B. Thereafter, the droplet 6B does not become resistant to the gravity, thus falling to the liquid surface 4. The solution 6 falls in one droplet 6C or in two droplets 6C when the droplet 6C which stays on the end 5 of the bar material 3 becomes too great. Supposing that the diameter of the bar material is Db, in terms of the length of the bar material, the volume of the droplet 6C is Db/15 when the droplet 6C is small or the volume of a column corresponding to Db/3 when the solution falls in two droplets 6C. Thus, when such a large quantity of droplet 6C with Cr content of (M) falls to the liquid surface 4, the density of Cr on the liquid surface 4 becomes high and the Cr content in vapor generated from the liquid surface 4 becomes greater than (M). This method has a problem in that the composition ratio among components of an alloyed thin film fluctuates in the travel direction of a substrate in forming a thin film by traveling the substrate above the crucible because the droplet 6C falls intermittently.